1. Field of the Invention
The present invention relates to a surface acoustic wave filter, a surface acoustic wave device including such a surface acoustic wave filter, and a communication device. More specifically, the present invention relates to a longitudinally coupled resonator type surface acoustic wave filter including a rotated Y-cut X-propagating LiTaO3 substrate as the piezoelectric substrate, and a surface acoustic wave device and a communication device including such a surface acoustic wave filter.
2. Description of the Related Art
Surface acoustic wave filters (hereinafter referred to as “SAW filters”) include a surface acoustic wave element utilizing a surface acoustic wave that propagates on the surface of the piezoelectric substrate. For example, in mobile communications devices, for example, portable telephones, SAW filters are used in various applications in radio frequency circuits.
In particular, since surface acoustic waves have a shorter wavelength than electromagnetic waves, surface acoustic wave devices, including surface acoustic wave filters, are more easily miniaturized. Thus, in recent years, in the communication device field, including portable telephones, which are required to be compact and low-profile, the demand for surface acoustic wave filters and surface acoustic wave devices are rapidly increasing.
Among SAW filters, in particular, longitudinally coupled resonator type surface acoustic wave filters (hereinafter simply referred to as “longitudinally coupled filters”) have been primarily used since they can be configured as low-loss radio frequency filters.
An example of a longitudinally coupled dual mode SAW filter is disclosed in Japanese Unexamined Patent Application Publication No. 5-267990.
The longitudinally coupled filter disclosed therein includes a group of three interdigital transducer electrodes (IDT electrodes) provided adjacent to one another on a piezoelectric substrate in the propagating direction of the surface acoustic wave, and reflectors arranged at two opposing ends of the group of interdigital transducer electrodes. The pitch of the innermost two electrode fingers of opposing IDT electrodes is determined on the basis of the wavelength λ of the surface acoustic wave. This achieves a fractional band of about 4% even in a radio frequency range approaching 1 GHz, and reduces the loss.
Meanwhile, longitudinally coupled filters, including the one disclosed in the publication No. 5-267990, require various filter characteristics depending upon their application. In particular, an RF filter for a remote keyless entry system requires a narrow-band characteristic such that it can be adjusted to a required passband width. In the past, in a longitudinally coupled filter for such an application, a crystal substrate having a temperature coefficient of zero has been used to achieve the narrow-band characteristic.
However, the crystal substrate has a small dielectric constant and has a small electromechanical coupling coefficient. Therefore, the impedance of the filter is high. Thus, an additional matching circuit is required for the longitudinally coupled filter. This arrangement, therefore, has some disadvantages during manufacturing, such as a more complicated configuration and an increased number of elements. Furthermore, since the resulting longitudinally coupled filter has greater loss, the quality of the filter is reduced.
Consequently, to achieve the narrow-band characteristic, recently, rotated Y-cut X-propagating LiTaO3 substrates have become more widely used as piezoelectric substrates. A longitudinally coupled filter using the rotated Y-cut X-propagating LiTaO3 substrate provides lower impedance than the filter using the crystal substrate, and overcomes the above-mentioned disadvantages.
However, when attempting to achieve a narrow-band characteristic with the longitudinally coupled filter using the rotated Y-cut X-propagating LiTaO3 substrate, the flatness is poor.
Specifically, as shown in FIG. 10, a known longitudinally coupled filter has a tapered waveform (indicated by the arrow) in the passband. Consequently, ripple deviation, which is an indicator of the flatness in the passband, increases, thereby causing a problem in that the width of the passband is reduced to less than the required width. This phenomenon where the width of the passband is reduced more than required will herein be referred to as an “excessive narrow-band phenomenon.”